[clang] [Clang] add wraps and no_wraps attributes (PR #115094)

[Justin Stitt via cfe-commits]

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@@ -433,6 +433,26 @@ Attribute Changes in Clang
 - Fix a bug where clang doesn't automatically apply the ``[[gsl::Owner]]`` or
   ``[[gsl::Pointer]]`` to STL explicit template specialization decls. (#GH109442)
 
+- Introduced ``__attribute__((wraps))`` which can be added to type or variable
+  declarations. Using an attributed type or variable in an arithmetic
+  expression will define the overflow behavior for that expression as having
+  two's complement wrap-around. These expressions will not be instrumented by
+  overflow sanitizers nor will they cause integer overflow warnings. They also
+  cannot be optimized away by some eager UB optimizations as the behavior of
+  the arithmetic is no longer "undefined".
+
+  There is also ``__attribute__((no_wraps))`` which can be added to types or
+  variable declarations. Types or variables with this attribute may be
+  instrumented by overflow sanitizers, if enabled. Note that this matches the
+  default behavior of integer types. So, in most cases, ``no_wraps`` serves
+  purely as an annotation to readers of code that a type or variable really
+  shouldn't wrap-around. ``__attribute__((no_wraps))`` has the most function
+  when paired with `Sanitizer Special Case Lists (SSCL)
+  <https://clang.llvm.org/docs/SanitizerSpecialCaseList.html>`_.
+
+  These attributes are only valid for C, as there are built-in language
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JustinStitt wrote:

No, it won't break the build. You will get an ignored attribute warning and the attributes won't be applied or have any effect. (I'll document this).

These attributes are relatively well supported in C++ but that language has many more ways for the attribute to disappear unexpectedly. 

This attribute is mostly needed in C (as there aren't any alternatives).

https://github.com/llvm/llvm-project/pull/115094